Allow to use labkit without relying on global state

Please see gitlab-com/gl-infra/readiness!46 (comment 425250320) for the background on the issue.

I suggest to make it possible to use Labkit with instances of "things" instead of global singletons. Please note that handbook recommends to "Avoid global variables, even in packages".

Here is a list:

• Tracing: !66 (closed)
• Sentry: !69 (closed) !75 (merged)
• Monitoring: !71 (merged)